C++: Linking issue when using dynamic libraries

[SonnleitnerF]

I am using MCAP within my codebase and have run into a linking issue when I am trying to include the reader/writer within header files. Until now I could prevent such issues by defining the MCAP_IMPLEMENTATION macro in the top level CMakeLists.txt and including the mcap reader/writer only in source files. Due to the nature of the code this is no longer possible as I am working with templates. A small excerpt of the linker error can be found below.

error LNK2005: "public: __cdecl mcap::MetadataIndex::MetadataIndex(struct mcap::Metadata const &,unsigned __int64)" (??0MetadataIndex@mcap@@QEAA@AEBUMetadata@1@_K@Z) already defined in *********.cpp.obj

I am using MSVC 17.9.7 with conan and cmake

I could not find any thing in the documentation how to fix this?

[indirectlylit]

Hi @SonnleitnerF -

We recently added a link in the docs to a community repo which can help smooth CMake integration: olympus-robotics/mcap_builder. In the readme, they note:

Do not add MCAP_IMPLEMENTATION macro, as that is handled for you by this wrapper.

Curious if this might help with your use-case?

[indirectlylit]

according to the readme, you should also be able to build it as part of your project:

https://github.com/olympus-robotics/mcap_builder?tab=readme-ov-file#build-mcap-locally

does that describe your use-case?

[SonnleitnerF]

This did not work unfortunately. I got some errors showing some incompatibility issues on windows with the C++ version i was using (C++20 with the C++23 features MSVC 17.9.7 supports). After that I did not further try this approach

[indirectlylit]

(cc @filippobrizzi - fyi)

[filippobrizzi]

unfortunately, I don't have a way to test on Windows to try and fix it, but @SonnleitnerF if you have time to contribute to olympus-robotics/mcap_builder that would be awesome for the next person with your same issue.

[SonnleitnerF]

I will keep it in mind! Thanks for the infos

[jtbandes]

MCAP_IMPLEMENTATION is supposed to be defined in only one translation unit, it should not be defined globally.

[SonnleitnerF]

what if I want to use it in different libraries though? Then I get "symbol unknown" linker errors, and if I declare it in every library I want to use it in, I get the mentioned "already defined symbol" linker error.

[SonnleitnerF]

I now resolved this issue by prebuilding mcap and linking against it. This way the macro is not needed anymore and no linker issues occur.

I leave the discussion open for other potential solutions, if other developers experiencing similar issues can have different approaches